1. Field of the Invention
This invention relates to the field of ultrasonic analysis of bone tissue in humans, and more particularly to a bone analysis apparatus as well as an improvement in the calibration and quality assurance of an ultrasonic bone analysis apparatus by using, for example, phantoms.
2. Description of the Related Art
The use of ultrasound in methods for detecting changes in bone characteristics is known. In particular, an ultrasound bone analysis apparatus has been used to analyze the properties of the heel bone or os calcis. The use of ultrasound is advantageous because it is non-invasive and is well-suited to repeated measurements or studies during medication since no ionizing radiation is used.
Precision and reliability of the ultrasonic bone analysis apparatus, as with other medical diagnostic instrumentation, are a matter of substantial importance. Therefore, the apparatus undergoes calibration and quality assurance regularly during its lifetime. Rather than using a human subject, the calibration and quality assurance is performed using a substitute medium that has specific ultrasonic properties. The calibration and quality assurance facilitate adjustment of the apparatus according to the specification of the instrument.
An ultrasonic bone analysis apparatus typically measures the rate of change of attenuation of ultrasound with frequency in the range of 200 to 600 kHz (xe2x80x9cbroadband ultrasound attenuationxe2x80x9d or xe2x80x9cBUAxe2x80x9d), and also the speed of passage of acoustic waves (xe2x80x9cspeed of soundxe2x80x9d or xe2x80x9cSOSxe2x80x9d) through the bone. The BUA is a relative quantity calculated using a baseline signal as a reference of the transmitted signal entering the bone.
The baseline is typically acquired by measuring the signal after passage through a reference medium. Because the reference signal is used to assess the transmitted signal, the reference medium should either minimally affect the ultrasonic signal or be well characterizable by, for example, having a known attention vs. frequency function.
Some existing ultrasonic bone analysis systems use a liquid as a coupling medium between the ultrasound transducers and the patient""s foot. These xe2x80x9cwet systemsxe2x80x9d require immersing the patient""s foot in a liquid bath in order to achieve acoustic coupling. These wet systems typically use, for example, water or water/saline solutions as the coupling medium.
However, since the SOS of water is temperature dependent, existing wet systems require the use of water heaters for heating the water to a predefined temperature at which the SOS is known. Accordingly, these systems are relatively costly to produce and operate. In addition, when a patient""s heel is placed in the water, the temperature of the water may vary, which can also vary the accuracy of the measurements.
For calibration and quality assurance of an ultrasonic bone analysis apparatus, phantoms are sometimes used. While some commercially available phantoms are suitable for monitoring temporal changes in scanner performance, the acoustic properties of these phantoms are typically significantly different from those of bones such as the os calcis. Therefore, these phantoms might not adequately mimic the human foot.
Heretofore, Clarke et al. proposed in xe2x80x9cA Phantom for Quantitative Ultrasound of Trabecular Bonexe2x80x9d, 39 Phys. Med. Biol. 1677-87, to use a phantom as a substitute medium in a wet system. The proposed phantom consists of a rectangular block manufactured from a mixture of liquid epoxy and gelatine particles. While the proposed phantom does have acoustic properties similar to bone and may be adequate for experimental purposes, Clarke et al. admit that the proposed phantom has a number of unsolved practical problems such as durability.
A phantom manufactured from an epoxy and glass bead mixture has also been used with a wet system. However, the manufacture of this phantom is believed to be complex and to require substantial supervision and control.
The measurement of SOS depends on the ambient conditions. Measuring accurately and comparing SOS data can be difficult due to the wide range of possible conditions, and such difficulties can be aggravated by imprecise control and determination of the conditions of the measurement.
Various media have been used for testing SOS measurements as well as for use as the acoustic coupling medium in the wet system as noted above. For example, pure water and saline solution of various sodium chloride concentrations have been employed. However, the SOS for each of these substances varies according to temperature, each substance having a positive temperature coefficient. Therefore, using one of these substances in the testing of the SOS measurements or when measuring the actual SOS through a patient""s heel, has the disadvantage that temperature is an additional variable.
A method of calibrating an ultrasound bone analysis apparatus having a plurality of transducer assemblies with a respective plurality of transducers and a respective plurality of coupling pads and measuring a SOS of a body part. The method comprises providing a plurality of coupling pads having a SOS substantially similar to the SOS of the body part to be analyzed. The plurality of transducer assemblies are adjusted until the plurality of coupling pads are mutually in contact, the plurality of coupling pads mutually contacting each other using a first amount of pressure. An ultrasound signal is transmitted through one of the plurality of transducers. A signal corresponding to the transmitted signal is received through another one of the plurality of transducers. A first propagation time of the transmitted signal and a first position of the transmitting and receiving transducers are determined. A body part is positioned between the plurality of coupling pads, the coupling pads contacting the body part using a second amount of pressure different than the first amount of pressure and an ultrasound signal is transmitted through the transmitting transducer. A signal corresponding to the transmitted signal is received through the receiving transducer and a second propagation time of the transmitted signal and a second position of the transmitting and receiving transducers are determined. A time for the ultrasound signal to pass from the transmitting transducer to the receiving transducer is determined based on the first and second propagation times and a width of the body part is determined based on the first and second positions. The step of positioning a body part between the plurality of coupling pads can include applying a non-aqueous gel between the body part to be analyzed and the coupling pads.
A method for calibrating an ultrasound bone analysis apparatus comprises providing a phantom having a frequency attenuation approximating a body part to be analyzed and having a predetermined speed of sound (SOS) and broadband ultrasound attenuation (BUA) measured at a predefined temperature. Calibration of the ultrasound bone analysis apparatus is performed at an arbitrary temperature, the calibration comprising, adjusting the plurality of transducer assemblies so that the plurality of coupling pads are mutually in contact, transmitting an ultrasound signal through one of the plurality of transducers, receiving a signal corresponding to the transmitted signal through another one of the plurality of transducers and determining a first propagation time of the transmitted signal and a first position of the transmitting and receiving transducers. The phantom is positioned between the plurality of coupling pads so that the coupling pads contact the phantom, an ultrasound signal is transmitted through the transmitting transducer, a signal corresponding to the transmitted signal is received through the receiving transducer and a second propagation time of the transmitted signal and a second position of the transmitting and receiving transducers are determined. A SOS of the ultrasound signal passing from the transmitting transducer to the receiving transducer based on the first and second propagation times and a width of the phantom based on the first and second positions are determined. The determined SOS is compared to the predetermined SOS of the phantom and based on the result of the comparison, an amount of change in BUA of the phantom is determined. A BUA reference curve and an additive constant are stored in memory based on the result of the comparison. The step of positioning the phantom between the plurality of coupling pads includes applying a non-aqueous gel between the phantom and the coupling pads. The additive constant may include a correction term derived from the result of the comparison performed in the comparison step. The additive constant includes the predetermined BUA measured at the predefined temperature.
A method is disclosed for performing a quality control evaluation on an ultrasound bone analysis apparatus. The method comprises determining a direction of change in BUA of a phantom by measuring a signal through the phantom and calculating BUA of the phantom and comparing to a previously determined BUA value for the phantom. A direction of change in SOS of the phantom is determined by measuring a signal through the phantom and calculating based thereon SOS through the phantom and comparing to a previously determined value for the phantom. An evaluation of the condition of the apparatus is performed based on the results of the determinations of the directions of change of the BUA and SOS. The evaluating step may comprise determining if the directions of change in SOS and BUA are in the same directions, wherein if the directions of change in SOS and BUA are not in the same directions, a malfunction of the apparatus is indicated to an operator.
A method is disclosed for evaluating parts of an ultrasonic bone analysis apparatus, comprising, performing quality control evaluations or calibration measurements on the apparatus utilizing a phantom, wherein during the evaluations or measurements, a width of the phantom is measured. The measured width of the phantom is at least periodically stored. A measured width of the phantom is compared to a stored width of the phantom, determining, based on the comparison, a variation in squish of the transducer pads. An evaluation of a condition of the transducer pads is made based on the determination.
A transducer assembly is disclosed for use in an ultrasound bone analysis apparatus, the transducer assembly comprising, a plurality of transducers, a plurality of coupling pads each of which covers at least a portion of a corresponding one of the plurality of transducers, and an acoustic coupling material provided between the transducer and the respective coupling pad, the acoustic coupling material comprising a material having a temperature independent speed of sound. The acoustic coupling material has a substantially zero temperature coefficient of sound propagation over a large range of temperatures. The acoustic coupling material may comprise a water and ethyl alcohol mixture, the mixture being 17% alcohol by weight.
An ultrasound bone analysis apparatus is disclosed comprising a pair of transducer assemblies, each comprising a transducer, a coupling pad which covers at least a portion of the transducer and an acoustic coupling material provided between the transducer and the respective coupling pad, the acoustic coupling material comprising a material having a temperature independent speed of sound, a time measuring device for measuring a propagation time of ultrasound based on a signal transmitted between said transducers, a distance measuring device for measuring at least one of a distance between the transducers and a width of a body part to be analyzed, and a processor for determining a speed of sound of a body part to be analyzed based on the measurements performed by said time measuring device and said distance measuring device. The apparatus may further comprise a unit for moving the transducer assemblies between various positions, including a position in which the coupling pads contact each other and a position in which the coupling pads contact a body part to be analyzed. The acoustic coupling material may have an SOS substantially similar to that of a body part to be analyzed. The apparatus may further comprise biasing units for biasing the pair of transducer assemblies such that the biasing units bias the coupling pads against each other at a first pressure and bias the coupling pads against the body part to be analyzed at a second pressure different than the first pressure.
An ultrasound bone analysis apparatus is disclosed comprising a container for containing an acoustic coupling material and capable of containing at least a portion of a body part to be analyzed within the acoustic coupling material, the acoustic coupling material comprising a material having a temperature independent speed of sound. The apparatus comprises a pair of transducer assemblies, each comprising a transducer and a coupling pad which covers at least a portion of the transducer. Each of the transducer assemblies is arranged on opposite sides of the container for transmitting and receiving ultrasound. A time measuring device measures a propagation time of ultrasound based on a signal transmitted between the transducers, and a processor determines a speed of sound of a body part to be analyzed based on the measurements performed by the time measuring device. The acoustic coupling material may have a substantially zero temperature coefficient of sound propagation over a large range of temperatures. The acoustic coupling material may comprise a water and ethyl alcohol mixture, the mixture being 17% alcohol by weight.